Upstream particles observed in the Earth's foreshock region

Abstract

On the basis of primarily an extensive study of fully three‐dimensional plasma data, we describe the interrelationships of the upstream particles and plasma waves observed in the earth’s foreshock region. The University of Iowa LEPEDEAS detect ions and electrons from 1 eV to 45 keV over all except ∼2% of the unit sphere. Comparisons are made with high time resolution particle data obtained by the University of California (Berkeley) instruments and plasma wave data collected by the University of Iowa plasma wave instruments on the two ISEE spacecraft. The presence of ion beams or dispersed ion distributions is found to be a sufficient condition for the presence of electrostatic and electromagnetic wave emissions. Detailed correlations of ions with plasma waves down to a tenth of an ion gyroperiod indicate that ion acoustic emission is enhanced when increased anisotropies and gyrophase organization are observed. Time aliasing effects limit the interpretation of velocity distributions taken within the foreshock region. High time resolution correlations between the different instruments, however, demonstrate that time variations of a single isotropic or anisotropic distribution cannot produce the dispersed ion distributions. Detailed analysis of high time resolution data reveals that the upstream particles undergo significant spatial and temporal variations including gyrophase organization. Gyrophase organization comprises groups of ions or ion clusters each one of which includes ions with similar pitch angles that gyrate together about a common guiding center. On the basis of our high time resolution analysis of three‐dimensional plasma data combined with magnetic field and plasma wave data, we conclude that (1) ions observed in the foreshock region display gyrophase organization produced by ion clusters with a spatial scale <1 Rg, and (2) dispersed ion distributions are produced primarily by direct sources at or near the bow shock.